This invention relates to a method and apparatus for diagnosing and monitoring a semiconductor based device, more particularly a surface photo-voltage in a substrate.
Surface photo-voltage principles are an important tool used for characterizing semiconductor materials. In particular, devices applying surface photo-voltage principles are becoming one of the main technologies for non-contact diagnostics and monitoring procedures used in many manufacturing processes for silicon based devices.
A surface photo-voltage measurement method is disclosed in U.S. Pat. No. 4,544,887. In this method, a beam of light is directed at a region of a surface of a specimen of semiconductor material and a photo-induced change in electrical potential at the surface is measured. The wavelength of the illuminating light beam is selected to be shorter than the wavelength of light corresponding to the energy gap of the semiconductor material undergoing testing. The intensity of the light beam is modulated, with both the amplitude of the light and the frequency of modulation being selected such that the resulting AC component of the induced photo-voltage is directly proportional to the intensity of light and inversely proportional to the frequency of modulation.
A surface photo-voltage measuring system can be used for non-contact diagnostics and monitoring of silicon based devices. The surface photo-voltage measurement system includes a surface photo-voltage measurement probe located above a measurement chuck. A wafer to be evaluated with the surface photo-voltage measurement system is placed on the measurement chuck.
A critical element in the use of the surface photo-voltage measurement system is to ensure that the surface photo-voltage measurement system is properly measuring the surface photo-voltage of the wafer. In the past, reference wafers have been used to calibrate photo-voltage measurement systems. However, reference wafers do not provide consistent surface photo-voltage measurement due to the sensitivity of the reference wafers to ambient conditions. There is a need in the art for a device that can consistently calibrate surface photo-voltage measurement systems.
The present invention relates to an apparatus for simulating a surface photo-voltage in a semiconductor wafer. In one embodiment the apparatus includes a photodiode, a resistor, and a disk. The photodiode has an anode and a cathode. The resistor has a first terminal and a second terminal. The disk has a first surface and a second surface. The resistor first terminal is in electrical communication with either the photodiode anode or the photodiode cathode. The resistor second terminal is in electrical communication with the first surface of the disk and the photodiode anode if the photodiode cathode is connected to the resistor first terminal, or the photodiode cathode if the photodiode anode is connected to the resistor first terminal.
The disk is made from a conductive or semiconductive material. The disk can also be process sized to duplicate the normal size and weight of industry standard semiconductor wafers to evaluate a surface photo-voltage measurement system used in production.
In one embodiment of the present invention, the photodiode is a planar photodiode. In another embodiment of the present invention, the photodiode is a photodiode array including a plurality of individual photodiode segments. In yet another embodiment of the present invention, the individual photodiode segments are in electrical communication with one or more electrical resistors.
In another embodiment of the present invention, the apparatus further includes an operational amplifier having an first input terminal, a second input terminal, and an output terminal. The first input terminal is in electrical communication with either the photodiode anode or cathode, the second input terminal is in electrical communication with the other of the photodiode anode and cathode, and the output terminal is in electrical communication with the resistor second terminal.
The present invention also relates to an apparatus for measuring a simulated photo-voltage in a semiconductor wafer. In one embodiment the apparatus includes a photodiode, a resistor and a disk used for simulating a surface photo-voltage and further adds a measurement chuck comprising a first surface, a surface photo-voltage measurement probe, and a light source for measuring the simulated surface photo-voltage. In another embodiment the light source generates a light having amplitude modulation.
Another embodiment of the present invention further adds an optional optical window including an electrically conductive transparent coating. The optical window is positioned adjacent to the photodiode and between the photodiode and the light source.
The present invention also relates to a method for simulating a surface photo-voltage. In one embodiment the method includes providing an apparatus for generating a simulated surface photo-voltage including a simulator disk, generating a light from a light source, illuminating the simulator disk, and generating a signal in the simulator disk. In another embodiment, the method further includes providing a measurement chuck and a surface photo-voltage measurement probe, and measuring the signal with the surface photo-voltage measurement probe. In another embodiment, the method further includes providing an operational amplifier.
The present invention also relates to an apparatus for simulating a surface photo-voltage including a photodiode, a disk, and a resistor. The photodiode has a anode metalization and a cathode metalization. The disk has a trench. The resistor has a first and second terminal wherein the resistor first terminal is attached to the photodiode cathode metalization with a first volume of conductive epoxy and the second resistor terminal is attached to the photodiode anode metalization with a second volume of conductive epoxy.
The present invention also relates to an apparatus for simulating a surface photo-voltage including a photodiode, a resistor, an operational amplifier, and a disk. The photodiode has an anode and a cathode. The resistor has a first terminal and a second terminal. The operational amplifier has a first input terminal, a second input terminal, and an output terminal. The disk has a first surface. The resistor first terminal and operational amplifier first input terminal are in electrical communication with either the photodiode anode or cathode. The operational amplifier second input terminal is in electrical communication with the other of the photodiode anode or cathode, and the operation amplifier output terminal is in electrical communication with the resistor second terminal. The photodiode is mounted to the disk first surface but the photodiode is not in electrical communication with the disk.